Another reason is; Lego isn't a 'safe' jungler. Almost all common jungle heroes have minions to absorb the damage for them, Lego doesn't have that, he has to take all the damage himself, which makes him far easier to gank. If you keep vision in their jungle, or roam around, you're likely to pick up free easy kills on him just by chance of him having low hp after a difficult camp or larger stack.

If there is not a better jungler left in the hero pool, then there is almost always great lanes for some form of 2-2-1.
Legionnaire is forced to farm until lvl 3/4.
He does not offer much to a push aside from clearing creep waves, which usually puts him in a bad position.
He can also be shut down relatively easily in a team fight until he gets a portal key.
However with all this, he does fit in some team line ups/situations, however as of late we have been running warbeast as a farming jungler instead of legionnaire.